Concept: off-the-grid sensor module Part 2

Well over a year ago I wrote a bit about off-the-grid sensor modules. TL;DR: combining an ATmega328 with an nRF24L01+ and using low power mode makes battery powered sensor modules very feasible. Adding a small solar panel and a LiPo battery with charger circuit makes it awesome.

At that time it was just an idea that wasn't that high in my things-to-do-list, but in the past few weeks that idea has become more relevant. In our office I've got a bunch of plants. For those interested, we're trying to grow pepper and paprika plants here, which is a big step up from the indestructible spider plants that are infecting the office building :) I've got over a dozen plants, some of which are not in my room, but I'd still like to keep an eye on them. Now I could just walk into the other room, check on the plants daily and say hi to my colleagues while I'm at it, but where's the fun in that?

Instead of some physical and social labor, I'd rather just have a dashboard to monitor the soil humidity of my plants, because that's what any other sane person would want, right? Besides, I have a Raspberry Pi sitting on my desk that's just begging to be used for something useful.

I initially thought about having a single Arduino Uno connected to a bunch of sensors and to the RPi using SPI, but that's going to get messy with all the wires so I got rid of that idea pretty quickly. Instead I'd rather have tiny sensor modules. There's no need to integrate the nRF24L01+ and the sensor in the design; those can be bought separately for way less money than it would cost to integrate them. Instead the module should just have a socket to plug one of those cheap nRF boards in and a connector for a sensor. And we're going to need power of course. Using an ATmega would be overkill for this kind of project. An ATtiny would be a better choice since it's smaller, uses less current and is cheaper, plus I've got a few in my parts bin.

So what do we need?

• connector for a nRF24L01+ module board (2x4 pins)
• 3.3V power supply for the nRF
• connector for an analog sensor (1x3 pins)
• programming interface connector (ICSP, 2x3 pins)

Since the module needs to be as small as possible I would like to use an ATtiny in a soic-8 package, those are about 5x5mm in size and have eight pins. Two of those pins are for VCC and GND, and a third one is used for RESET (required if you ever want to (re)program it. So that leaves 5 pins for other uses. Here's a pin layout of the ATtiny I have lying around:

And here's the pin layout for an nRF module:

That's a lot of pins:

1. GND
2. VCC (3.3V)
3. CE (Chip Enable), needs to be controlled
4. CSN (Chip Select, SPI), needs to be controlled
5. SCK (SPI Clock), needs to be controlled
6. MOSI (SPI Master Out, Slave In), needs to be controlled
7. MISO (SPI Master In, Slave Out), needs to be controlled
8. IRQ, we're not going to use this.

So we're going to need 5 pins just to control the nRF, occupying all the available pins on the mcu. Fortunately, Nerd Ralph has a brilliant solution to this problem, using just a few off-the-shelf components to multiplex the CSN and SCK pins. This frees up one pin on the mcu which we can now use for other purposes. Ralph also uses an LED to drop the voltage from 5V to roughly 3.3V. That's not something I'm going to use since I also want to be able to power the module from a 6V solar panel or a 3.7V LiPo. Instead I'm going to use a LP2985-N voltage regulator. It has a very low dropout voltage of max 300 mV @ 150 mA, meaning it can deliver 3.3Vout with just 3.6Vin. At 50 mA the dropout voltage is even less than half of that, making it suitable for running off a LiPo battery. Other advantages are that it comes in a very small 5 pin SOT-23 package and costs about $0.25. The ATtiny will also be powered by this regulator since it can't handle the 6V a solar panel can deliver.

Another thing Ralph did was tie CE to VCC. However, this means the nRF can no longer transition between RX and TX mode without powering down.

So, fire up KiCad, throw in all components, wire them together and start laying out the board:

Neat eh? That's just 32 by 11 millimeters, or 1.25 by 0.45 inches. So it's even smaller than the nRF board to be plugged in. The IC1 pads and silk screen look a bit weird, that's because it actually are two footprints on top of each other. The ATtiny13A comes in two SOIC packages: a small one and an even smaller one. This board accommodates for both packages.

The ADC pin can be used as a digital or analog input, or you could use it as a digital output (for controlling a relay for example). The ICSP connector allows for changing the firmware (remember to unplug the sensor and nRF in case your programmer uses 5V).
The bare material cost per board is about $2.70, an nRF module costs around $0.80 and a three pin soil hygrometer costs $1.35. So for less than $5 you've got a wireless module to monitor your plant!

It still needs power though, but I'll leave that topic for a future post.